Treating cattle with ivermectin: effects on the fauna and decomposition of dung pats

(1) The effects of a single therapeutic injection of the antiparasitic drug ivermectin (22,23-dihydroavermectin B1) administered to cattle at 200 μ g kg-1 bodyweight, under Danish conditions, were studied in field and laboratory experiments. (2) Faecally excreted ivermectin inhibited the development of larvae of dung-dwelling Diptera Cyclorrhapha in dung collected from cattle 0-30 days after treatment. Larvae of dung beetles (Aphodius spp.) were inhibited in dung from animals treated 1 day previously, and pupae and larvae of Diptera Nematocera were inhibited in dung from animals treated 1 and 1-10 days previously, respectively. Excreted ivermectin remained active against a laboratory strain of the housefly Musca domestica in dung pats exposed for 2 months in the field. (3) The decomposition of dung pats from recently treated heifers was delayed significantly when compared with untreated controls. No adverse effects of treatment were recorded on earthworms. Hence, the retarded decomposition rate was ascribed to the adverse effects on the primary dipteran decomposing fauna. (4) The consequences of treatment in terms of fouling of pastureland are discussed, and the need for further research on the implications of future routine use of continuous slow-release ivermectin treatments is emphasized.